Misfires'll sure cause vibes on a big single like ours.
The most common cause of misfires is tracking of the HV plug current to earth - usually caused by old insulation breaking down - or the spark plug being so worn and/or opened up that it's easier for the electricity to jump to earth than it is to jump the huge plug gap.
The easiest way to attempt to isolate tracking is to run the bike in a dark place - I do it in my garage, at night, with the lights off. Start the bike, and look for "fairy lights" - little sparkles that indicate electrical activity. Shine a torch on the sparks and you'll see where the tracking's happening.
Failing that, you can try re-routing the HT (Plug) lead slightly, along with spraying the ignition leads and electrics with WD40 or similar - but any spray-fix will likely be temporary.